The present invention relates to a helical stranded cable, particularly for mechanical motion transmission, which is composed of multiple strands or wires, helically wound around a common longitudinal axis, said cable being covered with an external jacket (2).
Such devices are well known in the art and widely used. While these devices satisfactorily serve their function, they still suffer from certain drawbacks.
Prior art stranded cables are typically composed of ten or more strands helically wound around a common longitudinal axis, to form the final cable. The great number of strands, or wires, involves a few drawbacks: first, the helicoid generated by the strands will have as many helical blades as strands, hence in prior art sheathed cables there will be at least as many points of tangency between the cable and the sheath as the strands or wires of the cable. Particularly, it shall be noted that in prior art cables, a greater number of strands or wires increasingly approximates a circumferential profile, hence the points of contact with the sheath associated to the cable increase with the number of strands, maintaining in any case a gap between two consecutive points of tangency of two different strands. The number of gaps increases (as the number of strand increases, and it has been found that the presence of those gaps together with the natural elasticity of the strands wounded in a cable and of the sheath leads to have one shock-like contact per one gap while the cable is moving inside the sheath for example while the cable is moving for mechanical motion transmission.
As to define the shock-like contact, the shock-like contact is that particular kind of contact that arises when two surfaces having different speed vectors became in contact each other. It has to be understood that effects of a shock-like contact becomes higher when the contact surface is smaller, as a matter of fact for the same condition of the speed vectors, if the contact surface or contact area is smaller and the force is the same, consequently the corresponding pressure in the contact area is higher. This leads furthermore to an higher wear in the contact surfaces. It has to be further noted that a punctual contact surface has the maximum of shock-like contact, having maximum pressure between the contacting surfaces and it implies maximum of wear in the contacting surface.
A number of drawbacks arise from this: first, a greater number of shock-like contacts between the cable and the sheath due to a greater points of contact, or so-called points of tangency, between the cable and the sheath, together with the presence of the aforementioned gaps, and natural elasticity, causes an increased wear of the cable and/or sheathe. Due to such wear, the cables usually have a reduced life, therefore they often have to be oversized to such an extent as to have a life suitable to the application for which they are designed.
Oversizing can be effected essentially in three possible ways: increasing the diameter of each strand, increasing the number of strands, or improving the mechanical properties of the cable material. These three typical arrangements involve three respective drawbacks: when the strand diameter is increased, the cable has a higher manufacturing cost and a heavier weight, and further requires a sheath of greater diameter, which increases the weight of the whole assembly. Therefore, the cable that is so oversized is not suitable for low-cost applications.
The arrangement with a greater number of strands involves the above mentioned drawbacks and makes the problem of the shock-like contact between the cable and the sheath even more noxious, by incrementing the number of gaps and therefore the number of shock-like contacts between the cable and the sheath, by incrementing the number of the punctual surfaces of the cable that came in contact with the sheath surface.
Furthermore the arrangement with a greater number of strands poses a problem associated to an increased flexibility of the cable, especially when subjected to pushing, whereby the cable is poorly effective in transmitting longitudinal efforts causing the cable to be pushed.
The prior art arrangement that provides an improvement of the mechanical properties of the cable allows to substantially keep the cable and sheath sizes unchanged, but is often rejected, because it causes an exponential increase of cable costs. Indeed, for driving signal transmission, the mechanical cable solution is usually preferred to the oil hydraulic solution, for a manufacturing and installation cost advantage, and in this arrangement the low cost advantage of the mechanical solution would be lost.
It shall be further noted that in prior art cables a lubricant is often used in combination with the cable jacket, to lubricate the inside of the cable thereby dramatically reducing friction between the cable and the sheath. The lubricant is often introduced from the end of the cable through which the cable is inserted in the sheath, so as to preserve the inside of the sheath from any infiltration of dust and/or dirt, which would further increase friction. Nevertheless, the lubricant in prior art cables cannot reach the very longitudinal center of the sheath and/or the end opposite the insertion end, because the lubricant is applied at a substantially very low pressure. Therefore, in prior art cables, the highest friction is found at the center point of the longitudinal extension of the sheath or at the end opposite the cable insertion end, which is therefore poorly lubricated.
A further prior art arrangement consists in covering the cable with intrinsically lubricating, or self-lubricating materials, such as PTFE (also known with the commercial name Teflon®). PTFE spreads over the inner surface of the sheath, due to friction, and keeps the cable lubricated. Nevertheless, this arrangement has a high implementation cost, due to the high cost of PTFE, and more generally of all self-lubricating materials, and is actually feasible only in few application cases. Furthermore, due to the high cost of self-lubricating materials, the surface cover made of these materials is very thin due to money saving requirements.
An object of this invention is to provide a helical stranded cable, particularly for mechanical motion transmission, which may simply and inexpensively obviate the drawbacks of prior art helical stranded cables particularly for mechanical motion transmission.
Furthermore prior art cable construction method provides the simple action of winding the strands helically around a common longitudinal axis, to form the final cable. Such a known construction method leads to form a cable that does not work efficiently in compression: the different strands, during compression tends to move away from each other and consequently the cable shows a sort of spring effect before transmitting the signal. The prior art cables made by the above mentioned simple winding method furthermore implies another related problem: the whole length of the cable is slightly shortened during the compression due to the spring effect.
Furthermore the prior art cable built by the prior art construction method presents another problem due to the fact that the strand wound together shows an exterior perimeter which has its profile made by the juxtaposition of the singles round profile of the different strands, this leading to have shock-like punctual contact between the cable and the sheath.
Another object of this invention is to provide a helical stranded cable production method, which may simply and inexpensively obviate the drawbacks of prior art helical stranded cables production method.